(1) Field of the Invention
This invention relates to combustors, and more particularly to combustors for gas turbine engines.
(2) Description of the Related Art
Gas turbine engine combustors may take several forms. An exemplary class of combustors features an annular combustion chamber having forward/upstream inlets for fuel and air and aft/downstream outlet for directing combustion products to the turbine section of the engine. An exemplary combustor features inboard and outboard walls extending aft from a forward bulkhead in which swirlers are mounted and through which fuel nozzles/injectors are accommodated for the introduction of inlet air and fuel. Exemplary walls are double structured, having an interior heat shield and an exterior shell. The heat shield may be formed in segments, for example, with each wall featuring an array of segments two or three segments longitudinally and 8-12 segments circumferentially. To cool the heat shield segments, air is introduced through apertures in the segments from exterior to interior. The apertures may be angled with respect to longitudinal and circumferential directions to produce film cooling along the interior surface with additional desired dynamic properties. This cooling air may be introduced through a space between the heat shield panel and the shell and, in turn, may be introduced to that space through apertures in the shell. Exemplary heat shield constructions are shown in U.S. Pat. Nos. 5,435,139 and 5,758,503. Exemplary film cooling panel apertures are shown in U.S. patent application Publication 2002/0116929A1 and Ser. No. 10/147,571, the disclosures of which are incorporated by reference as if set forth at length.
Exemplary combustors are operated in a rich-quench-lean (RQL) mode. In an exemplary RQL combustor, a portion of the fuel-air mixing and combustion occurs in an upstream portion of the combustor in which the fuel-air mixture is rich (i.e., the spatial average composition is greater than stoichiometric). In this portion of the combustor, the fuel from the nozzles mix with air from the swirlers and participative cooling air in the fore portion of the combustor. In an intermediate quench portion, additional air flow (“process air”) is introduced through orifices in the combustor walls to further mix with the fuel-air mixture and, over a short axial distance, transition the mixture to lean (i.e., less than stoichiometric) on a spatially averaged basis. This is often termed quenching of the reaction as, given typical fuel-air ratios, most of the energy in the fuel has been converted by reacting. In a downstream region, the mixture is lean and diluted to the design point overall fuel-air ratio as participative cooling further dilutes the mixture. An exemplary RQL combustor is shown in the aforementioned U.S. '929 publication.